A number of high-molecular weight glycoproteins on the membranes of murine lymphocytes have been described. These molecules include Ly-5 (T200) molecules, Lgp-100 molecules, Helix pomatia-binding glycoproteins, and Vicia villosa-binding proteins. All of these proteins are glycosylated; all except the V. villosa-binding protein are found on both T- and B-cells. Each molecule (except V. villosa) has at least two species of different apparent molecular weight, depending upon lymphocyte subpopulation. There are genetic and biochemical reasons for believing some of these proteins may be homologous. The aim of this study is therefore two-fold: 1) determine the biochemical basis for the differences in molecular weights of thymocyte, T-cell or B-cell forms of each of these molecules; 2) determine by biochemical comparison whether any of the four molecules (Ly-5, Lgp-100, H. pomatia-binding protein, and V. villosa-binding protein) is related to any of the others. Radiolabeled proteins will be isolated by immunoprecipitation or binding to lectins, and compared in their protein and carbohydrate moieties. It is hoped that further knowledge of the structure of these high-molecular weight differentiation molecules may lead to a better understanding of their role in the immune response.